The invention relates to a process, such as a well treating process, for dissolving siliceous materials, such as permeability-impairing particles of sand or clay in or around a well borehole. More particularly, the invention relates to dissolving such materials with a solution which can be flowed into contact with the siliceous material before the solution becomes strongly acidic and which can dissolve the siliceous material while maintaining a relatively high pH.
Numerous procedures for treating wells with siliceous-material-dissolving hydrofluoric acid solutions (commonly called "mud acid" solutions) have been previously proposed. For example, in U.S. Pat. No. 1,990,969 solutions of hydrochloric acid and water-soluble fluoride salts are used to avoid the surface handling of a hydrofluoric acid solution. In U.S. Pat. No. 2,001,579 the corrosivity of a solution of hydrochloric and hydrofluoric acids is reduced by incorporating a corrosion inhibitor. In U.S. Pat. No. 2,050,931 a water-wetting agent is injected ahead of an aqueous solution of hydrochloric and hydrofluoric acids that may be formed by reacting an aqueous solution or suspension of a fluoride salt with an emulsion in which the dispersed phase is an aqueous solution of hydrochloric acid. In U.S. Pat. No. 2,367,350 a slug of hydrochloric is injected ahead of a slug of hydrofluoric acid to remove multivalent cations from the region containing the siliceous material to be dissolved. In U.S. Pat. No. 2,663,689, boric acid is dissolved in an aqueous hydrochloric-hydrofluoric acid solution to avoid precipitation of insoluble fluoride salts, when multivalent cations are encountered, or precipitation of gelatinous hydrofluorosilicic acid, when the acidity of the solution is depleted.
U.S. Pat. No. 3,157,232 describes a well acidation process in which corrosion problems are avoided by forming the acid at the well site from non-corrosive chemicals. Formaldehyde is reacted with an ammonium salt of an acid (such as hydrochloric acid) to form the acid and the acid salt of hexamethylenetetramine. The R. E. Dilgren and R. E. Dilgren and F. M. Newman U.S. Pat. Nos. 3,215,199; 3,297,090 and 3,307,630 describe well acidization processes in which hydrolyzable organic halides are reacted in situ to form aqueous solutions of a hydrogen halide (such as hydrochloric acid). U.S. Pat. No. 3,630,285 describes a process for acidizing a reservoir having a temperature of at least 200.degree.F by injecting a water-soluble ester of an organic carboxylic acid that forms a water-soluble calcium salt. Such reactions might be used to form aqueous solutions containing hydrofluoric acid. However, the processes described in those patents were not proposed as "mud-acidization" processes for dissolving siliceous materials. Prior teachings and beliefs indicate that, in dissolving a siliceous material, an excess of a strong acid (such as hydrochloric acid) must be mixed with hydrofluoric acid in order to prevent the precipitation of insoluble or gelatinous salts of silicic or hydrofluorosilicic acids (e.g., see U.S. Pat. No. 1,990,969).